To provide a single representative measure of the aerodynamic performance of a ground vehicle with which the different configurations can be compared, a wind-averaged-drag coefficient ([WAC.sub.D]) can be defined so that, for a given ground speed ([U.sub.g]), it accounts for an equal probability of experiencing terrestrial winds from all directions.
For Canada and the United States, a typical mean terrestrial wind speed ([U.sub.avg]) used for these calculations is 11 km/h (7 mph ).
INFLUENCE OF TERRESTRIAL WINDS ON VEHICLE AERODYNAMICS
Terrestrial winds play an important role for the aerodynamics of ground vehicles.
Although the wind-averaged-drag approach accounts for variations in the mean terrestrial winds, turbulence associated with these winds and from the wakes of other vehicles on the road is usually not simulated in wind tunnel test programs.
They discuss that drag reductions for commercial vehicles have been shown to be smaller in turbulent flows than smooth flows, implying that it may be more difficult to reduce the drag of HDVs in flow conditions representative of the turbulent terrestrial winds experienced on the road.
Terrestrial winds play an important role for the aerodynamics of road vehicles, but are often not replicated in a sufficient manner in wind tunnels.
Several techniques have been used to simulate the fluctuations of terrestrial winds in a wind tunnel.
In the presence of terrestrial winds, the calibration of the on-board to the track-side measurement can only be done when the vehicle is in close proximity to the track-side anemometer of interest.
The main differences between the sites are the local vegetation, which affects the terrestrial winds above to the road surface, and the track/road surface, which may influence the rolling resistance.
However, one major deviation of the proposed rule from standard J2263 is that the terrestrial wind velocity measured by track-based anemometers at vehicle mid-height is used to "calibrate" the vehicle-mounted anemometer sensing the apparent wind at a different height.
Notwithstanding, the methods proposed in reference , in SAE standard J2263  and in the proposed phase-II rule  to name a few, rely on assumptions that do not take into consideration the turbulent nature of the terrestrial wind for the calibration of a vehicle-mounted anemometer.